1. Field of the Invention
The present invention relates to a printer that prints on single sheets, and to a printer control method.
2. Description of Related Art
Inkjet printers use a piezoelectric, thermal, or electrostatic actuator to discharge ink droplets to paper positioned on a platen, and thus print the discharged ink droplets on the paper. A roll paper holder or automatic sheet feeder (ASF) is usually attached to the inkjet printer, roll paper is supplied from the roll paper holder or single sheets are supplied from the ASF and conveyed in a particular direction, and these paper transportation and ink droplet discharge operations are controlled to print on the paper.
This operation is described in further detail next using an inkjet printer with an ASF by way of example. A pickup roller inside the ASF first picks and delivers a sheet from the ASF tray to the printer. Feed rollers then carry the paper to the printing area opposite the print head, and then advance the paper a specific distance at a specific interval in the paper transportation direction while ink droplets are discharged from the print head to the paper, thus recording letters, images, or other content on the paper. An inkjet printer with an ASF can thus continuously feed a plurality of sheets one at a time through the printing area inside the printer. See, for example, Japanese Unexamined Patent Application 2001-287427.
In order to feed paper from the ASF and position the paper relative to the print head, this type of inkjet printer typically uses two paper detectors to detect the leading edge and trailing edge positions of each sheet, and positions the paper to the print head based on the detected positions of the leading and trailing edges of the paper.
When the length of the paper transportation path in the printer is shorter than the length in the transportation direction of the paper used for printing, the paper will be positioned to the print head while the trailing edge of the paper remains inside the ASF, and printing may thus start while part of the paper remains inside the ASF. If printing starts while the trailing portion of the paper remains inside the ASF, the paper may be held in a large curl while printing continues depending on the position of the ASF relative to the printer (more specifically depending on the shape of the transportation path from where the paper is supplied from the ASF to the paper exit from the printer). Thus curling the paper increases the paper tension, and can thus adversely affect the feed pitch of the paper. Situations in which the paper cannot be advanced at the specified pitch and can thus only be fed at a smaller than expected pitch can even occur.
If the transportation length of the paper is shorter than the length of the paper transportation path in the printer, however, the paper is printed after the trailing edge of the paper is completely discharged from the ASF. In this case the printing paper is deposited flat on the paper transportation path from the leading edge to the trailing edge of the paper. The paper can therefore be fed and printed at the expected feed pitch because there is no additional tension on the paper.
When the leading end portion of the paper is printed while the trailing end is still held in the ASF but the trailing end portion of the paper is printed after the paper is completely discharged (freed) from the ASF, the feed pitch may vary between the leading and trailing ends of the same sheet of paper.
A further problem is that if the shape of the paper being printed differs according to the length of the paper in the transportation direction (the “transportation length” below), the feed pitch of the printer will also differ according to the shape of the paper being printed. Furthermore, a shift in the feed pitch also causes the dot pitch to vary in the paper transportation direction, thus causing the print quality to vary.